Over the last decade, cancer of the prostate has become the most commonly diagnosed malignancy among men and the second leading cause of male cancer deaths in the western population, following lung cancer (Landis et al., 1998, CA Cancer J. Clin. 48(1):6-29). Of all cancers, the incidence of prostate cancer increases most rapidly with age. As longevity among the western population increases, there continues to be a corresponding rise in the number of prostate cancers with an expected increase of 60% in this decade alone. Mortality has increased at a slower rate, but overall has doubled in the last 50 years. Although the disease is typically diagnosed in men over the age of 65, its impact is still significant in that the average life span of a man who dies from prostate cancer is reduced by 9-10 years. If discovered, early prostate cancer can now be cured with surgery in approximately 90% of cases. Unfortunately the disease is slowly fatal once the tumor spreads outsize the area of the gland and forms distant metastases. Early detection of the disease, while still confined to the prostate gland, and accurate staging for the selection of appropriate therapy should improve mortality rates.
Despite many advances in recent years, the precision with which an individual suffering from prostate cancer can be staged is still sub-optimal. The main reason for this is that tumor spread beyond the prostate is generally microscopic rather than macroscopic. Digital rectal examination of the prostate has been the cornerstone for the local staging of prostatic cancer for many decades, but it oftentimes underestimates the extent of the disease. Transrectal ultrasound by itself is only of limited value as a means of prostate cancer staging. Computer tomography and magnetic resonance imaging have generally been disappointing in the staging of prostate cancer (Kirby, 1997, Prostate cancer and Prostatic Diseases 1:2-10). Recent promising approaches to prostate cancer staging imply the use of biochemical and molecular technologies, centered around proteins or their corresponding nucleic acids which are preferentially expressed in prostate cells (Lange, 1997, In “Principles and Practice of Genitourinary Oncology” ed. Lippincott-Raven Publishers, Ch. 41, pp. 417-425). The most notorious prostate markers are PSA (prostate specific antigen) and PSM (prostate specific membrane) antigen.
PSA is a secreted glycoprotein encoded by the PSA gene located on chromosome 19. It is expressed under androgen control by glandular epithelial cells of the prostate and secreted into seminal plasma. PSA protein is normally confined to the prostate but in the case of prostatic disease such as cancer or BPH (benign prostate hyperplasia), PSA leaks into the blood where it is present in different forms, including one that is and one that is not bound to protein complexes (El-Shirbiny, 1994, Adv. Clin. Chem. 31:99). The measurement of total PSA serum concentrations is one of the most frequently used and FDA-approved biochemical tests in the screening and management of prostate cancer patients. Studies to date have suggested that screening with PSA, in conjunction with digital rectal exams and transrectal ultrasound, increases the detection of early prostate cancers often while still localized to the gland itself (Brawer et al., 1992, J. Urol. 147:841). Serum PSA is also useful for monitoring of patients after therapy, especially after surgical prostatectomy. However, total PSA measurements also identify a large number of patients with abnormally elevated levels who are subsequently found to have no prostate cancer. Recently, the concept of measuring the percent free/total PSA ratio was shown to increase the specificity of prostate cancer screening in men with PSA between 4 and 10 ng/mL (Letran et al., 1998, J. Urol. 160:426).
The PSM gene encodes a transmembrane glycoprotein expressed by epithelial cells of normal prostate, benign prostate hyperplasia and, to a greater extent, malignant prostatic tissue. Low levels of PSM are also detected in some other tissues (Israeli et al., 1994, Cancer Res. 54:1807). PSA and PSM have also been targets for molecular approaches to prostate cancer using RT-PCR (reverse transcription-polymerase chain reaction). This very sensitive nucleic acid amplification technology is used to identify cells based on the expression of specific messenger RNAs. It involves preparing RNA samples from tissues or body fluids, reverse transcribing it into cDNA and amplifying specific cDNAs by the use of primers that target the particular gene of interest. RT-PCR analyses of blood, lymph nodes and bone marrow from prostate cancer patients using PSA and PSM have disclosed the extreme sensitivity of this approach. However, the clinical value of molecular tests still has to be confirmed (Verkaik et al., 1997, Urol. Res. 25:373; Gomella et al., 1997, J. Urol. 158:326).
Thus, there remains a need to provide a more sensitive test for diagnosing prostate cancer. There also remains a need to provide a better test for the staging of prostate cancer. There also remains a need to provide a prostate cancer marker which is more specific and more reliable to prostate cancer detection, staging and treatment methods.
The present invention seeks to meet these and other needs.
A new prostate cancer marker, PCA3, was discovered a few years ago by differential display analysis intended to highlight genes associated with prostate cancer development (PCT application number PCT/CA98/00346). PCA3 is located on chromosome 9 and composed of four exons. It encodes at least four different transcripts which are generated by alternative splicing and polyadenylation. By RT-PCR analysis, PCA3 expression was found to be limited to the prostate and absent in all other tissues, including testis, ovary, breast and bladder. Northern blot analysis showed that PCA3 is highly expressed in the vast majority of prostate cancers examined (47 out of 50) whereas no or very low expression is detected in benign prostate hyperplasia or normal prostate cells from the same patients. There is at least 20-fold overexpression of PCA3 in prostatic carcinomas in comparison to normal or BPH tissues. PCA3 expression seems to increase with tumor grade and is detected in metastatic lesions.
In summary, prostate cancer staging based on specific markers such as PSA and PSM is a very promising avenue for the management of the disease. The drawback of using PSA or PSM for prostate cancer staging is that they are expressed in normal as well as in cancerous cells. In addition, poorly differentiated tumors may escape diagnosis since they tend to produce significantly less PSA protein than less aggressive tumors. This is the case for 10% of all prostate cancers. PCA3, on the other hand, is differentially expressed in cancerous and normal prostate cells, and its expression does not decrease with increasing tumor grade. PCA3 could therefore be a useful tool which may overcome the drawbacks of PSA and PSM in the diagnosis, staging and treatment of prostate cancer patients.
The present description refers to a number of documents, the content of which is herein incorporated by reference, in their entirety.